1. Field of the Invention
The present invention is directed to a circuit for detuning a resonator in a nuclear magnetic resonance imaging apparatus having an output connected to a terminal of a resonator via a high-frequency line composed of two lines, and wherein the resonator is shortened at its terminal with a shortening capacitor, which can be shorted for detuning the resonator.
2. Description of the Prior Art
In a nuclear magnetic resonance imaging apparatus (NMR apparatus), a magnetic high-frequency field is required in addition to the basic magnetic field and the gradient fields. The high-frequency fields are generated by resonators or antennas that can be classified as follows according to their function:
(1) The transmission antenna is also the reception antenna; substantially the entire body can be exposed to a high-frequency field with this antenna, which is therefore also called body resonator;
(2) The transmission antenna is the body resonator and the reception antenna is a special antenna that is smaller than the body resonator;
(3) The special antenna is the transmission and reception antenna.
Special antennas are employed to register tomograms of sub-areas of the human body such as, for example, tomograms of the knee, the head or the eye.
In order not to disturb the magnetic high-frequency field of the special antennas, the permanently integrated body resonator must be detuned when the special antennas are activated. Body resonators often have their length shortened at their terminals or feed points by shorting capacitors, which are then short circuited via switches for detuning. This solution requires high-frequency switches that must be directly arranged at the resonators. In addition to the high-frequency lines, therefore, control lines for the high-frequency switches are also conducted to the resonators in this type of known circuit.
A detuning circuit disclosed in U.S. Pat. No. 4,801,885 wherein the input of the high-frequency lines to the resonators is short-circuited for detuning the resonators. The short-circuit is effected by a quarter-wave transmission line connected to the input of the high-frequency line, this quarter-wave transmission line being operated no load at its other end for detuning. As used herein, .lambda. stands for the wavelength of an electromagnetic wave having the operating frequency of the nuclear magnetic resonance imaging apparatus on the line. In addition to the high-frequency lines whose length must be exactly equal to the length of the resonators in order to avoid phase errors, further line sections are required whose length must be exactly .lambda./4. This requires high-precision manufacture of both the high-frequency line the quarter-wave transmission line.
The magnetic high-frequency field should be substantially uniform in the nuclear magnetic resonance imaging apparatus. The body resonator is therefore composed of two resonators lying opposite one another that are fed by high-frequency signals that are phase-shifted by 180.degree. relative to one another. The structure of body resonators and the in-phase feeding of the high-frequency signals is disclosed by German Published Application 31 33 432. The manner by which a transmission signal generated by a transmitter can be uniformly divided onto the two resonators lying opposite one another is also disclosed therein. In a first version, the feed points of the two resonators lie opposite one another at identical ends. The 180.degree. phase shift is achieved by an intervening conductor section having a length of .lambda./2. In another version, the current is supplied proceeding from different sides. In this case, the leads for the two resonators lying opposite one another must be of the same length. The employment of relatively long conductor sections is disadvantageous in these two versions.
A balancing circuit at whose output two signals out of phase by 180.degree. and being symmetrical relative to a reference potential are present is known from FIG. 2 of the article by Bottomley et al., "Human in Vivo Phosphate Metabolite Imaging with 31P NMR". This article is published in Magnetic Resonance in Medicine 7, 1988, pages 319-336. The balancing circuit therein is referred to as a "balun" and is used to connect a balanced-to-ground resonator in an NMR apparatus to an unbalanced-to-ground high-frequency line, for example a coaxial line.